hotend backpressure

hi all, have seen this term mentioned a couple of times - could somebody please explain what it is
thank you roger.

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Prusa 'Explorer' (3dStuffMaker), GEN6, J-head Mk III-B, Bowden Extruder, Marlin 1.0.0 RC2, Repitier-Host V0.84 and Slic3r 0.9.8, PLA. Live at Victoria, Australia.

In simple terms volume in = volume out, but you have to push that volume through the nozzle exit hole, which results in pressure build up insider he nozzle, that pressure also pushes back on the incoming filament. At some point, the incoming filament can't overcome the force required to extrude the plastic, and either the stepper skips or the Hobbes bolt mangles the filament.

Hotends with wider nozzles, or shorter nozzles (the final diameter hole) will require less force to extrude.

It 's somewhat important to understand that changes in pressure aren't instantaneous, and again shorter or narrower exit holes will show less hysteresis in changes in pressure, say transitioning from not printing to printing, or changing the speed at which the Hotend is moving.

Shorter exit holes aren't always a win since it generally means the Hotend is more likely to drip or ooze plastic on travel moves.

Just as I hit the reply button Polygonhell's post shows up.

Coulden't say it better myself.

so if i am having some problems getting good flow at the start, then having a fast first layer would be putting more pressure on the
system - more likely to have a back pressure problem. ????

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Prusa 'Explorer' (3dStuffMaker), GEN6, J-head Mk III-B, Bowden Extruder, Marlin 1.0.0 RC2, Repitier-Host V0.84 and Slic3r 0.9.8, PLA. Live at Victoria, Australia.

Why don't you try describing the problem or better yet posting pictures.
Yes in principle increasing speed increases back pressure, it's more of an issue on the first layer because most people actually print it lower than the othe layers, which increases the pressure further.
Are you losing steps?
Stripping filament?
Getting some sort of Jam?

what kind of hotend have you got?
you shouldn't really have any issues at startup, if anything the back pressure is roughly proportional to the amount of force behind the filament

ideally what you want in a hotend is a short transition zone and a short meltzone, the brass and ptfe hotends have the problem where the meltzone and transition zone changes length depending on how fast the filament is going through it,

so essentially the material is solid as close to the meltzone as possible and the length of it that is soft is a small as possible, most of the all metal hotends achieve this much better than the brass and ptfe hotends,

polygonhell - my problems is as describer in "back to crap" post - wanted to know what back pressure was on the side.
basically i think i am printing too much too close to the bed - maybe causing these back pressure problems - what would the
resultant problems of backpressure look like, how long does it take to be a problem?

jolly, still trying to find out from my supplier what sort of hot end i have. mostly covered in kapton tape . it has a heat sink. there is a picture of it in the other post.

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Prusa 'Explorer' (3dStuffMaker), GEN6, J-head Mk III-B, Bowden Extruder, Marlin 1.0.0 RC2, Repitier-Host V0.84 and Slic3r 0.9.8, PLA. Live at Victoria, Australia.

Usually, when I've go my nozzle too close to the bed or told Slic3r the bottom layer was too thick, the bottom layer looks more like a smeared on mess than neat slightly flattened lines of filament. With obvious tool marks where the head crosses the fill.

If I'm filling a large enough area, the pressure builds up and the bold strips my filament. At this point the fill will trail off.

Sometimes, this doesn't happen until the second layer. Some pressuer was released printing the second layer edges, but the lower fill is so rough and sticking up that the pressure starts building again. Same result.

is it summer or winter over there?
what you'll find is that hotend has a long meltzone and the pla is getting soft a little bit higher up than it should be, and plugging it up a bit, what also can happen is that the large retraction distances mean than the softer pla may be getting pulled into the coldzone and plugging it up there

i would start lowering the retraction numbers put a fan on the cold end, and maybe run the temperature a little lower on hotter days,

komb, then what do you do - go higher? 0.1, 0.2?

jolly, it is summer (no actually still spring) but was 31C on sat (dry heat).
to tell you the truth i haven't calibrated my machine fully yetl - it came partially assembled and i think they did most of that - was running pretty good.
i am on a tight deadline to get some things printed but will calibrate it properly after that - promise.
ive noticed it does dribble a bit when moving from one fill to the next but not enough to worry me at this stage (need practical parts not artistic at the moment).
i believe the retraction is set up for more isn't it for a bowden setup??? so may indeed be retracting more than normal. not sure what it is at the moment.
have said in the other post that i just changed fans so that might not be cooling the nozzle as well (I thought the fan was just to cool the plastic )
.

i think i recall seeing some wade extruder where there is a fan up the top - is that cooling the coldend?

Edited 1 time(s). Last edit at 11/25/2012 10:33PM by rogerw.

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Prusa 'Explorer' (3dStuffMaker), GEN6, J-head Mk III-B, Bowden Extruder, Marlin 1.0.0 RC2, Repitier-Host V0.84 and Slic3r 0.9.8, PLA. Live at Victoria, Australia.

rogerw Wrote:
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> komb, then what do you do - go higher? 0.1, 0.2?
>

I usually do one of two things.

If it's real bad:

1. I pause the print.
2. remove the stuff on the plate.
3. Get the filament moving through the head again.
4. Move the Z stop up a tad. (can be tricky)
5. Restart print.

If perimeter not too squashed, let it go otherwise repeat.
Leave Z stop there for next print.

Unless I messed up first layer height in Slic3r. Then fix that first and restart whole procedure.

If I'm feeling Bad... Push filament till it starts filling again.

This is tricky as the machine is still moving and leaves a gap in the print.
Only do this for test and throw away or personal prints. Never production prints.

Actually best not to do that anyway.